Conventional electropolishing (EP) is a common metal finishing process in which the metal object to be polished (the “work-piece”) is connected to the positive terminal of a DC electrical power supply as the anode. The surface of the work-piece or some portion of that surface is brought into contact with an electrolyte solution, which, in turn, also contacts a second electrode (the cathode), which is connected to the negative terminal of the power supply. When a suitable voltage difference is applied between the two electrodes, or a suitable current density is established at the anode, then the work-piece undergoes anodic oxidation forming solvated metal ions that dissolve into the electrolyte. Within a certain range of operating conditions this occurs in such a way that rough areas of the surface become smoother. In this context, current density is defined as the electrical current per unit surface area of cathode in contact with the electrolyte and has units such as mA/cm2.
EP is capable of producing a mirror-like reflective finish on many different kinds of metals and is especially useful for polishing metal parts with curved surfaces and complicated shapes. Recent interest has focused on the use of EP to selectively remove excess copper in the fabrication of integrated circuits via the copper damascene process.
Planarization and removal of excess copper from Cu damascene wafers is currently achieved by chemical-mechanical polishing (CMP), involving mechanical abrasion and chemical reactions with oxidizers and other chemicals. However, CMP is a costly process, generates hazardous waste products, and is incompatible with the mechanically fragile materials currently under development for improved dielectric layers.
Alternate approaches to planarization have been disclosed by S. Mazur et al., (co-pending applications U.S. Ser. No. 60/546,192; U.S. Ser. No. 60/546,198; U.S. Ser. No. 60/611,699; and U.S. Ser. No. 60/570,967) and by S. Dabrai et al. (U.S. Pat. No. 6,722,950). These references are incorporated herein in their entirety.
While these methods provide improvements over conventional CMP and standard EP, there remains a need for a planarization process that is effective over a wide range of operating conditions. In particular, it is of interest to maximize the current density while minimizing the voltage, and to reduce the sensitivity of the current density to interfacial velocity and to interfacial pressure between the membrane and the work-piece.